Is the galaxy full of Earths?

Scientists announced in June 2013 that three planets orbiting star Gliese 667C could be habitable. This is an artist's impression of the view from one of those planets, looking toward the parent star in the center. The other two stars in the system are visible to the right.

This diagram shows the planets thought to orbit star Gliese 667C, where c, f, and e appear to be capable of having liquid water. The relative sizes, but not relative separations, are shown to scale.

This diagram lines up planets recently discovered by Kepler in terms of their sizes, compared to Earth. Kepler-22b was announced in December 2011; the three Super-Earths were announced April 18, 2013. All of them could potentially host life, but we do not yet know anything definitive about their compositions or atmosphere.

This illustration depicts Kepler-62e, a planet in the habitable zone of a star smaller and cooler than the sun. It is located about 1,200 light-years from Earth in the constellation Lyra.

This illustration depicts Kepler 62f, a planet in the habitable zone of a star smaller and cooler than the sun, in the same system as Kepler 62e.

This diagram compares the planets of our own inner solar system to Kepler-62, a five-planet system about 1,200 light-years from Earth. Kepler-62e and Kepler-62f are thought capable of hosting life.

The planet Kepler-69c is located about 2,700 light-years from Earth in the constellation Cygnus. This is an illustration of the planet, which is the smallest yet found to orbit in the habitable zone of a sun-like star.

This diagram compares the planets of our own inner solar system to Kepler-69, which hosts a planet Kepler-69c that appears to be capable of hosting life, in addition to planet Kepler-69b.

This artist's illustration represents the variety of planets being detected by NASA's Kepler spacecraft.

• NASA's Kepler telescope discovered a planet that is 10% larger than Earth


• Jim Bell: The new planet, 186f, may or may not be inhabitable


• He says there appears to be a large number of Earth-like extrasolar planets


• Bell: Discovery supports the idea that habitable worlds are probably common

Editor's note: Jim Bell is a professor in the School of Earth and Space Exploration at Arizona State University and a member of the NASA Curiosity Mars rover camera team. He is president of The Planetary Society and author of, most recently, "The Space Book: 250 Milestones in the History of Space and Astronomy." The opinions expressed in this commentary are solely those of the author.

(CNN) -- The amazing discoveries from NASA's Kepler planet-hunting space telescope keep rolling in. The latest, announced this week by astronomers, is the discovery of a planet just 10% larger than the Earth orbiting in the so-called "habitable zone" of the star Kepler-186.

In our solar system, Earth is the only planet in the habitable zone -- the distance from the sun where liquid water can exist on the surface without boiling away (like on Venus), or turning to ice (like on Mars).

The new planet, imaginatively dubbed Kepler-186f for now, appears to be in the same kind of Goldilocks place in its solar system. Not too hot, not too cold, just right. 186f could be the closest planet yet found.

It could be Earth 2.0. Maybe.

The Kepler telescope can detect planets like 186f and tell us their size, but it can't tell us what they're made of or what they're like. Is 186f a rocky planet like the Earth with a thin atmosphere and oceans and continents? Or does it have a thick atmosphere and a small rocky core? Or is it a big metallic body with no atmosphere at all? Or something else entirely?

Over the past decade a veritable zoo of planets has been discovered around other stars using a variety of telescopic methods, from hot Jupiters (giant planets close in to their star) to super Earths (rocky worlds many times the size of our planet). Add to that the amazing diversity of planets and moons that we've discovered right here in our own solar system during the past four decades from the Voyager spacecraft and other missions, the variety is astounding.

Even if 186f doesn't turn out to be Earth-like, the number of actual Earth-like extrasolar planets out there appears to be staggering. During its four-year mission, Kepler observed just a tiny, random, average piece of the sky, one you would cover with your fist held at arm's length. More than 1,000 planets have been discovered so far from just the nearby stars in that tiny patch of the sky.

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186f, for example, is "just" 500 light years away -- a veritable next-door neighbor on the galactic scale. If you extend the results of that little survey across the entire, 100,000-light-year-wide Milky Way galaxy, you end up concluding that there are likely to be tens of billions of Earth-sized planets in our galaxy alone. And many of them must be orbiting in their sun's habitable zones as well.

Astronomers are scrambling to use other telescopes, on the ground and in space, to try to figure out what 186f and the thousand other new worlds discovered so far are really like. And new space observatories are being planned to try to follow up and expand on the results from the Kepler mission (which stopped collecting new planet data last year).

The implications of what they find could be profound, especially if they're able to detect an atmosphere there, and -- the Holy Grail -- especially if that atmosphere contains telltale gases like water vapor, oxygen, or methane, key indicators that the place may be habitable.

Just because a planet is in a star's habitable zone, though, and just because it has an environment that is potentially habitable, doesn't mean that planet is necessarily inhabited. But there are astronomers looking out for that possibility, too.

It's no coincidence, for example, that the lead author of the study that discovered 186f, Dr. Elisa Quintana, is from a research organization called the Search for Extraterrestrial Intelligence, or SETI, institute. I'm sure other SETI astronomers are making extra efforts to train their radio telescopes on Kepler-186 and those other recently discovered exoplanet systems, to listen for any stray signals.

Or, perhaps, they'll find a targeted signal, a cosmic "hello?" beamed our way by our neighboring astronomers on 186f, who are also trying desperately to answer the question, "Are we alone?"

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